1. Field of the Invention
This invention relates to optical circulators, for example as are suitable for use in coupling to free space optical communications systems.
2. Description of the Related Art
An optical circulator is a basic building block for optical systems. An optical circulator is a nonreciprocal device. An optical signal that enters port 1 of the optical circulator exits from port 2, but an optical signal that enters port 2 does not exit from port 1. It exits from a different port 3 instead.
Because of their nonreciprocal nature, optical circulators have widespread application. For example, optical circulators are commonly used in fiber optic communications networks for wavelength routing and optical signal retrieval, such as wavelength division multiplexing (WDM). Optical circulators are also used in conjunction with erbium doped fiber amplifiers (EDFA), Raman amplifiers, optical add/drop multiplexers (OADM), dispersion compensators and optical time domain reflectometers (OTDR). Whenever an optical signal enters an optical component by one optical path and exits from the component by counter-propagating along the same optical path, an optical circulator is a candidate for separating the entering and exiting optical signals.
However, most, if not all, optical circulators that are used in fiber optic communications networks are symmetric. All of the ports typically are single mode fibers. Along the entering optical path, the optical circulator couples light from the port 1 single mode fiber to the port 2 single mode fiber. The light typically then propagates through some additional components and returns to the circulator propagating in the opposite direction along the port 2 single mode fiber. The circulator couples the returning light to the exit path along the port 3 single mode fiber. The symmetric design is generally preferred for fiber optic communications networks because single mode fibers are generally preferred due to their performance.
However, this approach is not suitable for all applications. For example, in a free space optical communications system, the light that is transmitted from a transceiver may have a well-defined Gaussian shape and therefore may arrive via a single mode fiber. However, due to effects such as aberrations along the free space optical communications link and overfilling of the receive aperture, the light received by a transceiver typically does not have such a tightly controlled mode and cannot be efficiently coupled back into a single mode fiber.
Therefore, there is a need for optical circulators that can accommodate different incoming and outgoing optical signals.